"The method could apply equally well to commercial aluminum airframe fuselage skins or to transportation infrastructures such as bridges and railways for subways and trains," says technique developer Douglas Adams, assistant professor of mechanical engineering at Purdue.

The method lets researchers automatically diagnose the structural integrity of materials made from layers of ceramics, plastics, metal alloys, and fabrics, and held together by a gluelike matrix. Strong, lightweight materials such as these are finding use in missiles, aircraft, and other weapons systems, including a new type of armor in future tanks.

The new composite armor is said to be much more effective than its metal counterpart, but with one hitch: It doesn't show outside structural damage even if the inside is seriously harmed. Purdue's vibration technique would come in handy here because it can constantly check the condition of the composite armor and send a warning if the material is ready to fail.

The technique has proven sensitive enough to detect damage from small impacts that might be incurred when, say, a wrench hits the material. Tests on composite parts take place on a diagnostic system that uses a series of vibrating actuators and sensors. The actuators transmit high-frequency acoustical waves that hit material defects and scatter back toward the transmitting source, where sensors pick them up. "How that scatter is distributed tells us how big the damage site is, and where it is," says Adams.

Similar diagnostic methods have been tried in the past but without much success: Too many embedded sensors and actuators throughout a part weakened the composite material. Purdue says the key to its technique is a relatively meager array of actuators and sensors on the perimeter of a structure. "Our sparse arrays do no harm, which is the first requirement for any structural health monitoring system. They are also much easier to maintain than a widely embedded array if a transducer happens to fail," Adams explains.